The structure and integrity of DNA is of immense biological and biomedical importance. It is therefore essential to identify and to characterize the enzymes that alter DNA structure. For instance, DNA helicases are a large and diverse group of ATP-driven motor proteins that unwind DNA and participate in a broad range of biological processes. The study of helicases has been and will continue to be a critically important area of research. The human HARP protein (also known as SMARCAL1 and ATP-dependent ATPase A) was recently found to be an ATP-driven annealing helicase - that is, a novel enzyme that catalyzes the rewinding of complementary ssDNA that is bound by replication protein A, the major ssDNA- binding protein in the nucleus. Thus, HARP is a motor protein that acts in the opposite manner as DNA helicases. HARP is a distant member of the SNF2 family of ATP-dependent motor proteins. Mutations in the HARP gene are responsible for a rare autosomal recessive pleiotropic disorder known as Schimke immuno-osseous dysplasia (SIOD). The available data suggest that HARP contributes to the maintenance of genome integrity, and that SIOD is caused by a deficiency in the annealing helicase activity of HARP. More recent studies have led to the discovery of a second annealing helicase, termed AH2, for annealing helicase 2. AH2 is a novel protein that has not yet been reported in the literature. Unlike HARP, AH2 does not bind to RPA. It therefore appears likely that HARP and AH2 will have distinct biological functions. The long-term objective of this project is to determine the factors and biological phenomena that are associated with annealing helicases. The specific aims are as follows: to investigate the function of AH2 as well as the relation between AH2 and HARP; to develop new assays for annealing helicase activity and to analyze the effects of annealing helicases on different biochemical and biological processes; and to search for new annealing helicases, which includes DNA annealing helicases as well as DNA-RNA annealing helicases. These studies should provide novel and important advances in this emerging area of biomedical research.